---

name: nginx description: | Nginx reverse proxy and web server configuration skill.

USE WHEN:

• Configuring Nginx as a reverse proxy in front of application servers (Node.js, Python, Ruby, Java)
• Setting up HTTPS/TLS termination with certificates
• Configuring virtual hosts, upstream load balancing, and rate limiting
• Optimising static file serving, gzip compression, and cache headers
• Adding security headers (HSTS, CSP, X-Frame-Options, X-Content-Type-Options)
• Setting up WebSocket proxying
• Diagnosing 502, 504, 413, or SSL handshake errors

DO NOT USE FOR:

• Application-level logic (belongs in the app server, not Nginx)
• Container orchestration routing (use Kubernetes Ingress or a service mesh skill instead)
• Full API gateway features (rate-limit-by-user, auth, JWT — use api-gateway skill)
• Apache httpd configuration allowed-tools: Read, Grep, Glob, Write, Edit, Bash

---

Nginx — Reverse Proxy & Web Server

Core Concepts

Nginx uses a master/worker process model. The master reads config and manages workers; workers handle connections. The event loop in each worker is non-blocking, so a single worker can handle thousands of concurrent connections.

Key config hierarchy:

http { ... }                # Global HTTP settings
  upstream backend { ... }  # Pool of app servers
  server { ... }            # Virtual host (vhost)
    location / { ... }      # URI matching block

Nginx evaluates server blocks by listen port and server_name. Inside a server, location blocks are matched in order: exact (=), prefix longest-match (^~), regex (~, ~*), then implicit prefix.

---

Production HTTPS Server Block (Full Template)

# /etc/nginx/sites-available/myapp.conf

# --- Upstream pool -----------------------------------------------------------
upstream app_backend {
    least_conn;                        # Route to least-busy worker
    server 127.0.0.1:3000;
    server 127.0.0.1:3001;
    keepalive 64;                      # Persistent connections to upstream
}

# --- Rate limiting zones (defined in http{} context in nginx.conf) -----------
# limit_req_zone $binary_remote_addr zone=api:10m rate=20r/s;
# limit_req_zone $binary_remote_addr zone=login:10m rate=5r/m;

# --- HTTP → HTTPS redirect ---------------------------------------------------
server {
    listen 80;
    listen [::]:80;
    server_name example.com www.example.com;

    # Allow Let's Encrypt ACME challenge
    location /.well-known/acme-challenge/ {
        root /var/www/certbot;
    }

    location / {
        return 301 https://$host$request_uri;
    }
}

# --- Main HTTPS server -------------------------------------------------------
server {
    listen 443 ssl;
    listen [::]:443 ssl;
    http2 on;                          # Nginx ≥ 1.25.1 directive (older: listen 443 ssl http2)
    server_name example.com www.example.com;

    # --- TLS -----------------------------------------------------------------
    ssl_certificate     /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
    ssl_trusted_certificate /etc/letsencrypt/live/example.com/chain.pem;

    ssl_protocols       TLSv1.2 TLSv1.3;
    ssl_ciphers         ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256;
    ssl_prefer_server_ciphers off;     # Let TLS 1.3 clients pick

    ssl_session_cache   shared:SSL:10m;
    ssl_session_timeout 1d;
    ssl_session_tickets off;           # Disable for forward secrecy

    ssl_stapling        on;
    ssl_stapling_verify on;
    resolver            1.1.1.1 8.8.8.8 valid=300s;
    resolver_timeout    5s;

    ssl_dhparam         /etc/nginx/ssl/dhparam.pem;  # openssl dhparam -out dhparam.pem 2048

    # --- Security headers ----------------------------------------------------
    add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;
    add_header X-Frame-Options           "SAMEORIGIN" always;
    add_header X-Content-Type-Options    "nosniff" always;
    add_header X-XSS-Protection          "1; mode=block" always;
    add_header Referrer-Policy           "strict-origin-when-cross-origin" always;
    add_header Permissions-Policy        "geolocation=(), microphone=(), camera=()" always;
    add_header Content-Security-Policy   "default-src 'self'; script-src 'self' 'unsafe-inline' https://cdn.example.com; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self'; connect-src 'self' https://api.example.com; frame-ancestors 'none';" always;

    # --- Gzip ----------------------------------------------------------------
    gzip              on;
    gzip_vary         on;
    gzip_proxied      any;
    gzip_comp_level   6;
    gzip_types        text/plain text/css text/xml text/javascript
                      application/json application/javascript application/xml+rss
                      application/atom+xml image/svg+xml;
    gzip_min_length   1024;

    # --- Logging -------------------------------------------------------------
    access_log /var/log/nginx/myapp.access.log combined buffer=4k flush=5s;
    error_log  /var/log/nginx/myapp.error.log warn;

    # --- Static assets with long-lived cache ---------------------------------
    location /static/ {
        alias /var/www/myapp/static/;
        expires 1y;
        add_header Cache-Control "public, immutable";
        access_log off;
    }

    # --- Rate-limited API endpoints ------------------------------------------
    location /api/auth/ {
        limit_req zone=login burst=10 nodelay;
        limit_req_status 429;
        proxy_pass http://app_backend;
        include /etc/nginx/proxy_params;
    }

    location /api/ {
        limit_req zone=api burst=50 nodelay;
        limit_req_status 429;
        proxy_pass http://app_backend;
        include /etc/nginx/proxy_params;
    }

    # --- WebSocket endpoint --------------------------------------------------
    location /ws/ {
        proxy_pass         http://app_backend;
        proxy_http_version 1.1;
        proxy_set_header   Upgrade $http_upgrade;
        proxy_set_header   Connection "upgrade";
        proxy_set_header   Host $host;
        proxy_set_header   X-Real-IP $remote_addr;
        proxy_read_timeout 3600s;      # Keep WS connections open
        proxy_send_timeout 3600s;
    }

    # --- Default proxy -------------------------------------------------------
    location / {
        proxy_pass http://app_backend;
        include /etc/nginx/proxy_params;
    }

    # --- Error pages ---------------------------------------------------------
    error_page 404             /404.html;
    error_page 500 502 503 504 /50x.html;

    location = /404.html {
        root /var/www/myapp/errors;
        internal;
    }

    location = /50x.html {
        root /var/www/myapp/errors;
        internal;
    }
}

---

/etc/nginx/proxy_params (shared include file)

proxy_http_version 1.1;
proxy_set_header Host              $host;
proxy_set_header X-Real-IP         $remote_addr;
proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header Connection        "";   # Required for keepalive upstream
proxy_connect_timeout 10s;
proxy_send_timeout    60s;
proxy_read_timeout    60s;
proxy_buffering       on;
proxy_buffer_size     8k;
proxy_buffers         8 8k;

---

Worker / Global Tuning (/etc/nginx/nginx.conf)

user  www-data;
worker_processes      auto;           # One per CPU core
worker_rlimit_nofile  65535;          # Match system ulimit -n

error_log  /var/log/nginx/error.log warn;
pid        /var/run/nginx.pid;

events {
    worker_connections  4096;         # Max connections per worker
    multi_accept        on;
    use                 epoll;        # Linux only; Nginx selects automatically
}

http {
    sendfile            on;
    tcp_nopush          on;           # Batch send headers + start of file
    tcp_nodelay         on;           # Disable Nagle for keepalive connections
    keepalive_timeout   75s;
    keepalive_requests  1000;

    server_tokens       off;          # Don't reveal Nginx version in headers

    client_max_body_size 50m;         # Increase if file uploads are needed
    client_body_timeout  30s;
    client_header_timeout 30s;

    # Rate limit zones (referenced in server blocks)
    limit_req_zone $binary_remote_addr zone=api:10m   rate=20r/s;
    limit_req_zone $binary_remote_addr zone=login:10m rate=5r/m;
    limit_conn_zone $binary_remote_addr zone=addr:10m;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;   # Not text/html — avoids MIME sniffing

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}

---

Rate Limiting with Zones

# Zone definition (http context):
limit_req_zone $binary_remote_addr zone=api:10m rate=20r/s;

# Usage in location:
location /api/ {
    limit_req zone=api burst=100 nodelay;
    # burst=100 → allow up to 100 requests queued above rate
    # nodelay  → don't delay burst requests, reject immediately after burst
    limit_req_status 429;
    proxy_pass http://app_backend;
}

$binary_remote_addr uses 4 bytes (IPv4) or 16 bytes (IPv6) — much smaller than $remote_addr string, allowing more entries per MB of zone memory.

---

Named Locations and try_files

# Serve SPA with fallback to index.html
location / {
    root   /var/www/myapp/dist;
    try_files $uri $uri/ @index;
}

location @index {
    root /var/www/myapp/dist;
    add_header Cache-Control "no-cache";
    try_files /index.html =404;
}

# PHP-FPM example (for reference)
location ~ \.php$ {
    fastcgi_pass   unix:/run/php/php8.2-fpm.sock;
    fastcgi_index  index.php;
    include        fastcgi_params;
    fastcgi_param  SCRIPT_FILENAME $document_root$fastcgi_script_name;
}

---

Custom Log Format

# In http context
log_format main_ext '$remote_addr - $remote_user [$time_local] '
                    '"$request" $status $body_bytes_sent '
                    '"$http_referer" "$http_user_agent" '
                    'rt=$request_time uct=$upstream_connect_time '
                    'uht=$upstream_header_time urt=$upstream_response_time '
                    'cs=$upstream_cache_status';

access_log /var/log/nginx/access.log main_ext buffer=16k flush=10s;

---

Common Commands

# Test config syntax before applying
nginx -t
nginx -T                             # Also dumps parsed config

# Reload without dropping connections
nginx -s reload

# Graceful shutdown (drain connections)
nginx -s quit

# Force stop
nginx -s stop

# Check Nginx version and compile-time modules
nginx -V 2>&1 | tr ' ' '\n'

# Follow logs
journalctl -u nginx -f
tail -f /var/log/nginx/myapp.access.log

# Check which process is bound to port 443
ss -tlnp | grep :443

# Inspect open file descriptors for Nginx workers
cat /proc/$(pgrep -o nginx)/limits | grep 'open files'

# Reload systemd unit and restart
systemctl daemon-reload && systemctl restart nginx

# Enable / disable site
ln -s /etc/nginx/sites-available/myapp.conf /etc/nginx/sites-enabled/
unlink /etc/nginx/sites-enabled/myapp.conf

---

Anti-Patterns

Anti-pattern	Why it's harmful	Fix
default_type text/html in http {}	Causes browsers to execute unknown files as HTML — XSS vector	Set default_type application/octet-stream
No keepalive in upstream block	New TCP connection per request to backend; high latency and FD usage	Add keepalive 64; and proxy_set_header Connection ""
Missing worker_rlimit_nofile	Nginx workers hit OS default (1024) FD limit under load → silent failures	Set worker_rlimit_nofile 65535; and raise worker_connections accordingly
Logging to /var/log/nginx/access.log without buffer	Synchronous disk writes per request; degrades throughput by 10–30% under load	Add buffer=16k flush=10s to access_log
No proxy_read_timeout	Uses Nginx default (60s); long-running API calls get killed silently	Set explicitly: proxy_read_timeout 120s; on affected locations
server_tokens on (default)	Exposes exact Nginx version in headers; aids targeted vulnerability scanning	Set server_tokens off; in http {}
add_header inside nested location without always	Headers dropped on 4xx/5xx responses — security headers missing on error pages	Always use add_header ... always;
No client_max_body_size increase for upload endpoints	Default is 1m; any file upload > 1 MB returns 413	Raise to appropriate limit per location or globally
Using if blocks for proxy logic	if in Nginx is "evil" — can cause subtle bugs; context-dependent behaviour	Use map, limit_except, or separate location blocks
Wildcard server_name *	Matches all unintended hostnames; can serve private APIs to wrong vhosts	Use explicit names; set a catch-all default server that returns 444

---

Troubleshooting

Symptom	Likely cause	Diagnostic / Fix
502 Bad Gateway	Upstream app not running or listening on wrong port	ss -tlnp | grep 3000; check app logs; verify proxy_pass URL
504 Gateway Timeout	App is running but too slow to respond	Increase proxy_read_timeout; profile app; check DB queries
413 Request Entity Too Large	client_max_body_size too low	Raise limit in location or globally; check upload endpoint
SSL handshake errors (in client)	Expired cert, wrong chain, TLS version mismatch	openssl s_client -connect host:443 -servername host; check ssl_trusted_certificate
Mixed content warnings	Backend returning HTTP links despite HTTPS termination	Set proxy_set_header X-Forwarded-Proto $scheme; and configure app to trust it
HSTS blocks HTTP access after config rollback	Browser has cached Strict-Transport-Security	Cannot be immediately undone; see ssl-tls skill HSTS section
WebSocket connections drop after 60s	Default proxy_read_timeout	Set proxy_read_timeout 3600s; in the WebSocket location
Rate limit affects legitimate users	Zone too small or rate too low; burst not set	Increase zone size, raise rate, add burst parameter; whitelist internal IPs with geo
Static files returning 403	Directory or file permissions incorrect	ls -la /var/www/myapp/static; ensure www-data can read; check alias vs root
Nginx not reloading after systemctl reload	Syntax error in new config	Always run nginx -t before reloading; check journalctl -u nginx -n 50